1. Field of the Invention
The present invention relates to a power transmission device for driving a robot joint and the like.
2. Description of the Related Art
Hitherto, in a robot, such as an industrial robot, the amount of displacement of a joint has generally been controlled to a desired value by position control (servo control). A power transmission mechanism of a joint of this type of robot adopts, in many cases, a highly rigid structure that minimizes the chance of a change in the amount of displacement of the joint, which is caused by a change in an external force, so as to make it possible to accurately control the amount of displacement of the joint to the desired value.
However, the joint having the power transmission mechanism of the highly rigid structure exhibits poor flexibility under a variety of external environments that make it difficult to identify or predict the position or shape of an external object or a disturbance or the like beforehand. If, for example, a driven member of a joint comes in contact with an unexpected external object, then a situation frequently occurs, in which it becomes difficult to properly move the driven member or an excessive external force acts on an actuator that drives the joint.
In recent years, therefore, a study on a robot having a joint that permits elastic displacement has been carried out by the present applicant and others.
Meanwhile, as a power transmission mechanism of a joint of an industrial robot or the like, there has conventionally been known one having a construction proposed in, for example, Japanese Patent No. 4442464 (hereinafter referred to as Patent Document 1). According to the construction of the power transmission mechanism, a wire between two pulleys is provided with a spring so as to transmit power between the two pulleys through an elastic force generated by the spring, thereby enabling the joint to be elastically displaced.
For a robot required to carry out, for example, various tasks under various environments, the power transmission mechanism of a joint is desired to be configured to allow the stiffness characteristics (the degree of stiffness) of the joint to be variably controlled, thereby permitting a flexible motion of the robot suited for each type of task under each environmental condition.
It is further desired to configure the power transmission mechanism of the joint to allow the joint to generate a viscous force as necessary and to allow the viscosity characteristics, namely, the degree of ease of generating a viscous force, to be controlled.
However, the power transmission device having the construction described in the foregoing Patent Document 1 merely has the spring installed on the wire between the two pulleys. Hence, the power transmission device is not adapted to allow the stiffness characteristics between the two pulleys, i.e., the stiffness characteristics of the joint, to be variably controlled and also not adapted to allow the viscosity characteristics between the two pulleys to be variably controlled. There has been a risk that the power transmission device constructed as described in Patent Document 1 would inevitably have a complicated structure, an increased size or deteriorated drive efficiency if an attempt were to be made to enable the power transmission device to variably control the stiffness characteristics between the two pulleys, i.e., the stiffness characteristics of the joint, or to variably control the viscosity characteristics between the two pulleys, i.e., the viscosity characteristics of the joint.